New Spin-Polarized Electron Source Based on Alkali Antimonide Photocathode

New spin-dependent photoemission properties of alkali antimonide semiconductor cathodes are predicted based on the detected optical spin orientation effect and DFT band structure calculations. Using these results, the ${\mathrm{Na}}_2\mathrm{KSb}/{\mathrm{Cs}}_3\mathrm{Sb}$ heterostructure is designed as a spin-polarized electron source in combination with the ${\mathrm{Al}}_{0.11}{\mathrm{Ga}}_{0.89}\mathrm{As}$ target as a spin detector with spatial resolution. In the ${\mathrm{Na}}_2\mathrm{KSb}/{\mathrm{Cs}}_3\mathrm{Sb}$ photocathode, spin-dependent photoemission properties were established through detection of a high degree of photoluminescence polarization and high polarization of the photoemitted electrons. It was found that the multi-alkali photocathode can provide electron beams with emittance very close to the limits imposed by the electron thermal energy. The vacuum tablet-type sources of spin-polarized electrons have been proposed for accelerators, which can exclude the construction of the photocathode growth chambers for photoinjectors.

Figure 1

Calculated bulk electronic band structures of (a) ${\mathrm{Na}}_2\mathrm{KSb}$ and (b) GaAs. Gray lines show spectra calculated without SOI included. Colored dots show orbital character of the states (see keys).

Figure 2

Experimental setup. (a) Schematic drawing of optical setup for spatial CL polarization detection with a compact vacuum photodiode for the spin polarization measurements. Electrons emitted by $\hslash \nu$-photon excitation from the ${\mathrm{Na}}_2\mathrm{KSb}$ source are injected into an ${\mathrm{Al}}_{0.11}{\mathrm{Ga}}_{0.89}\mathrm{As}$ target. (b) Image of CL intensity distribution in the anode ${\mathrm{Al}}_{0.11}{\mathrm{Ga}}_{0.89}\mathrm{As}$ structure under injection of 2 eV electrons emitted from the photocathode at room temperature. (c) Cross-sectional view and a photograph of the assembled photodiode from the ${\mathrm{Na}}_2\mathrm{KSb}$ photocathode side, which can be used as a tablet-type source of spin-polarized electrons for accelerators. (d) Quantum yield as a function of the incident photon energy and (e) photoluminescence spectra from ${\mathrm{Na}}_2\mathrm{KSb}$ photocathode excited with the 1.59 eV (780 nm) laser diode. (f) Energy band diagram of both the ${\mathrm{Na}}_2\mathrm{KSb}$ source and the ${\mathrm{Al}}_{0.11}{\mathrm{Ga}}_{0.89}\mathrm{As}$ target with the NEA separated by a vacuum gap. $E_c$ is the conduction band, $E_v$ is the valence band and $E_{\mathrm{F}}$ is the Fermi level. (g) Circularly polarized $({\sigma }^{+},{\sigma }^{-})$ components of the PL spectra of ${\mathrm{Al}}_{0.11}{\mathrm{Ga}}_{0.89}\mathrm{As}$ target excited with the circular polarized light with 1.67 eV (740 nm) photon energy. (h) The corresponding circular polarization degree of the PL emission determined as $P_{\mathrm{PL}}=(I_{{\sigma }^{+}}-I_{{\sigma }^{-}})/(I_{{\sigma }^{+}}+I_{{\sigma }^{-}})$.

Figure 3

Circularly polarized (${\sigma }^{+}$, ${\sigma }^{-}$) components of the PL spectra of ${\mathrm{Na}}_2\mathrm{KSb}$ photocathode excited with the circular polarized emission of laser diode with (a) 1.49 eV (780 nm) and (b) 3.05 eV (405 nm) excitations. The peaks at 1.42 eV in panels (a) and (b) correspond to the ${\mathrm{Na}}_2\mathrm{KSb}$ band gap transition. (c) and (d) The corresponding circular polarization degree of the PL emission determined as $P_{\mathrm{PL}}=(I_{{\sigma }^{+}}-I_{{\sigma }^{-}})/(I_{{\sigma }^{+}}+I_{{\sigma }^{-}})$. (e) The circular polarization degree of the PL emission of ${\mathrm{Na}}_2\mathrm{KSb}$ compared with GaAs ($0.5\mu \mathrm{m}$ thickness) as a function of photon excitation energy.

Figure 4

(a) Circularly polarized (${\sigma }^{+}$, ${\sigma }^{-}$) components of the CL spectra excited by the injection of spin polarized electrons emitted from the ${\mathrm{Na}}_2\mathrm{KSb}/{\mathrm{Cs}}_3\mathrm{Sb}$ cathode at the accelerating voltage of 1.0 V. (b) The circular polarization degree of the CL emission determined as $P_{\mathrm{CL}}=(I_z^{\uparrow }-I_z^{\downarrow })/(I_z^{\uparrow }+I_z^{\downarrow })$. (c) Image of the spin-integrated CL intensity, (d) difference between spin-up and spin-down CL intensities ($I_z^{\uparrow }-I_z^{\downarrow }$), and (e) 2D distribution of the derived CL circular polarization degree $P_z$. The images were taken at an accelerating voltage of 1 V. (f) Left axis: dependence of the CL circular polarization degree on the energy of injected spin-polarized electrons for the ${\mathrm{Na}}_2\mathrm{KSb}$ photocathode compared with the $\mathrm{GaAs}/(\mathrm{Cs},\mathrm{O})$ photocathode where a similar ${\mathrm{Al}}_{0.11}{\mathrm{Ga}}_{0.89}\mathrm{As}/(\mathrm{Cs},\mathrm{O})$ spin detector is used. Right axis: Dependence of the integrated nonpolarized CL (logarithmic scale) on the energy of injected electrons (green squares).